1. Field of the Invention
The present invention relates to a touch panel which is formed using a touch sensor and a method for driving the touch panel. In particular, the present invention relates to a touch panel in which pixels provided with touch sensors are arranged in matrix and a method for driving the touch panel. Further, the present invention relates to a semiconductor device and an electronic device including the touch sensor.
2. Description of the Related Art
In recent years, a display device provided with a touch sensor has attracted attention. The display device provided with a touch sensor is called a touch panel, a touch screen, or the like (hereinafter referred to as a touch panel). Examples of the touch sensor include a resistive touch sensor and a capacitive touch sensor, which are different in operation principle. In both kinds of sensors, data can be input by contact of an object to be detected with a display portion of a display device, or the like.
As one of such touch sensors, an optical touch sensor is given. In an optical touch panel, a touch sensor which detects light is provided, and a display screen also serves as an input region. As an example of a device including such an optical touch sensor, a display device having an image capturing function, which is provided with a contact area sensor which captures an image, is given (for example, Patent Document 1). In the optical touch panel, light is emitted from the touch panel. In the case where an object to be detected exists at a given position of the touch panel, light at the region where the object to be detected exists is blocked by the object to be detected, and part of light is reflected. A photo sensor (also referred to as a photoelectric conversion element) which can detect light is provided in a pixel of the touch panel, and the photo sensor detects the reflected light, so that existence of the object to be detected in the region can be recognized.
In addition, it has been attempted to give a personal authentication function or the like to an electronic device typified by a portable information terminal such as a mobile phone (for example, Patent Document 2). A finger print, a face, a hand print, a palm print, a pattern of a hand vein, and the like are used for personal authentication. In the case where a portion different from the display portion has a personal authentication function, the number of components is increased, and the weight or price of the electronic device could possibly be increased. Patent Document 3 discloses a technique by which a fingertip is detected in accordance with the brightness of outside light.